1. Field of the Invention
This invention relates generally to the treatment of water, and in particular to treatment of water with high total dissolved solids, such as water used in conjunction with oilfield services. For example, the invention relates to treating oily flow-back or produced water from an oil and gas well prior to disposal or for reuse as injection or fracture flow-back water.
2. Background Art
Fracture flow-back water and water produced from a well may contain hydrocarbons, solids, bacteria, and heavy metals. Produced water is generated in the process of lifting oil and gas from subterranean formations that also include water. Water accompanies oil and gas as it is lifted to the surface. Reinjection of this produced water improves the recoverable reserves from a reservoir by establishing an external water drive and by maintaining reservoir pressure. Produced water may also be reused to fracture formations.
Produced or flow-back water must be cleaned for reuse or for disposal. Often, it is preferable to treat such water at the well site to reduce the costs of handling and transporting large volumes of water to a remote treatment site. The treatment of wastewater may involve many processes, depending on the characteristics and quality of the influent feed water and requirements for the effluent treated water.
One common wastewater treatment process is the removal of various dissolved metals, heavy metals, minerals, or other compounds that are present in the water in ionic form. Water softening, the removal of principally calcium and magnesium ions, is one common application. A well-known removal process is ion exchange, in which the water is passed through a resin bed, and certain ions in solution are preferentially sorbed by and replaced with ions from the resin. Another such removal process is lime-soda softening, in which slaked lime and soda ash are added to the water. These reagents react with various metallic ions in solution to form metallic hydroxides and carbonates. These reagents also raise the pH of the water, which advantageously corresponds to a range of low solubility for the metallic hydroxides and carbonates. As a result, the addition of slaked lime and soda ash causes the precipitation of the metals from solution. Moreover, the precipitate act as coagulants, thus enmeshing suspended particles for easier removal, as described below.
Another, almost universal, wastewater treatment process is clarification, in which stable colloidal solids that will not effectively settle by the force of gravity and slow-settling suspended solids are removed from the water. In most common systems, clarification is a three-step process. The first step, coagulation, is the addition and rapid intense mixing of a coagulant that destabilizes the suspended solids to create micro-particles. The complex chemistry of coagulation and charge destabilization need not be described in detail here. Common coagulants include aluminum sulfate, ferrous sulfate, ferric sulfate, ferric chloride, lime, soda ash, and polyelectrolytes. The next step, flocculation, is the slow stirring or gentle agitation of the water to aggregate the micro-particles into larger, rapid-settling flocs. The final step is separation of the flocs from the water. Sedimentation is a commonly used process in which flocs settle to the bottom under the force of gravity. Centrifugation may also be used for separation, but is not commonly used for high-flow wastewater treatment systems.
To date, wastewater treatment systems have generally evolved in the context of municipal sewage treatment and industrial wastewater treatment. The oilfield industry has substantially adopted these conventional treatment systems, which until recently, proved adequate for oilfield use.
Conventional wastewater treatment facilities, such as used for municipal sewer and water systems and the like, commonly employ gravity-induced sedimentation. That method is preferred in order to limit the energy required for water treatment. As a result, clarifiers are constructed with large flocculation pools and large graduated settling basins to provide a required system flow rate. Conventional clarifiers are described, for example, in U.S. Pat. No. 7,258,788 issued to Pollock on Aug. 21, 2007, entitled “Circular Clarifier Apparatus and Method,” and U.S. Pat. No. 4,054,514 issued to Oltmann on Oct. 18, 1977, entitled, “Sedimentation Apparatus with Flocculating Feed Well,” both of which are incorporated herein by reference.
FIG. 1 illustrates a wastewater treatment system (300) commonly employed for oilfield use. The system (300) employs a horizontal basin (302) that is open to the atmosphere. Basin (302) includes three interior vertical bulkheads (304) that divide it into four compartments—one each for rapid mixing (306), precipitation (308), flocculation (310), and settling (312). Influent wastewater (350) flows into the rapid mixing compartment (302) at one end of the basin (302). Treated water (352) flows out of the top of the settling compartment (312) at the opposite side of basin (302), with the heights of bulkheads (304) determining the water levels in all four compartments. A sludge line (314) also exits the bottom of settling compartment (312). A sludge rake or auger system (360) may be provided to aid in removing sludge from the bottom of the settling tank (312). Sludge waste may subsequently be centrifuged for thickening.
In the rapid mixing compartment (306), coagulant (354) such as lime, is added to begin the treatment process. Soda ash and caustic soda may also be added. Water then flows over the first bulkhead (304) into the precipitation compartment (308) A cylindrical mixing chamber (316) is located within the precipitation chamber (308). The mixing chamber (316) draws fluid in at the top and expels it at the bottom, creating radial up-flow in the precipitation chamber (308) outside of the mixing chamber (316). The mixing enhances crystal formation. Soda ash and/or caustic soda (356) may be added to the precipitation chamber (308). A sludge pump (315) and recirculation line (318) provides recycled sludge to enhance precipitation.
The water next flows over the second bulkhead (304) into the flocculation chamber (310). In the flocculation chamber (310), polyelectrolytes (358) are added, and gentle mixing enables floc to build. From the flocculation chamber (310), water flows over the third bulkhead (304) and is diverted to the bottom of the settling chamber (312) by a weir (319). A parallel arrangement of lamellar fins (320) provides for rapid settling of suspended solids as the water flows upward. Clarified water exits the settling chamber (312) via a series of collection troughs (322).
Maintaining the proper steady-state levels within conventional prior art systems is critical for successful operation. A disadvantage of conventional prior art systems is that should an equipment casualty result in improper levels, it may be necessary to shut down, drain, and clean the entire system to restore proper operation. Such repair is time-consuming and costly. In addition, the slow and unpleasant drain and clean process is required every time the system is relocated. It is desirable to have a self-draining, self-cleaning water treatment system that is easy to relocate.
Another disadvantage of prior art systems is their large footprint. The flow capacity of prior art systems is directly related to the area occupied by the settling tanks. Because real estate at an oil and gas well site may be severely limited, it is desirable to provide a water treatment system that both removes dissolved metals and clarifies the water that has a reduced footprint. Accordingly, the oil and gas industry requires water treatment systems with small footprints customized to its unique requirements.
Solid-contact clarifier units, also known as up-flow clarifiers, are also known in the art. Such units combine mixing, flocculation and sedimentation into a single structural unit for a reduced footprint. Mixing and flocculation occur in one compartment, and the flocculated water flows through a sludge blanket to effect floc removal by solid contact with the floating sludge blanket. Nevertheless, solid-contact clarifiers still require a relatively large sludge blanket surface area to accommodate a required system flow rate, and are therefore still not well-suited for oilfield use.
A further disadvantage of prior art clarifiers is the requirement for polymers. Synthetic polymeric coagulants, or polyelectrolytes (e.g., polyacrylates, polymaleates and their copolymers and phosphonates), are widely used in oil and gas water treatment processes for clarification. However, polyelectrolytes have low biodegradability. Additionally, although less expensive than inorganic metal coagulants such as aluminum or ferric salts, polyelectrolytes are significantly more costly than lye and soda ash. Accordingly, a polymer-free wastewater clarification process that uses only low cost and readily available coagulants and reagents is preferable.
Perhaps the most significant issue with the prior art water treatment systems is their inefficiency in treating water of the quality of now becoming prevalent in the oilfield. Municipal and industrial wastewater is typically characterized by low total dissolved solids (“TDS”), for example, between 500-1000 ppm. Accordingly, conventional prior art systems were not designed for water with high TDS. However, due to growing production of shale formations that require artificial fracturing, there is now a concomitant need to treat large volumes of frac flow-back water. Frac flow-back water often has high TDS, for example, in the range of 30,000 ppm. Typical prior art systems cannot handle the TDS content, and flow must be throttled back to about twenty percent of their nameplate capacity in order to keep the settling chamber from overloading. And, inadvertent overloading of the settling chamber requires shutdown and cleaning of the system, further exacerbating an already low throughput. Accordingly, a water treatment system suitable to handle high TDS feed without requiring even greater area for settling tanks is highly desirable.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a method and apparatus for effective and efficient treatment of water having high total dissolved solids content.
Another object of the invention is to provide a method and apparatus for water treatment that uses induced gravity for separation, thereby providing a significantly reduced footprint.
Another object of the invention is to provide a method and apparatus for water treatment having a single vessel for precipitation, coagulation, and flocculation, thereby providing a reduced footprint.
Another object of the invention is to provide a method and apparatus for water treatment that clarifies water without the use of polyelectrolytes.
Another object of the invention is to provide a method and portable apparatus for water treatment that is self-draining and self-cleaning, thereby facilitating shut-down and relocation of the system.
Another object of the invention is to provide a method and portable apparatus for water treatment that is mounted to a trailer for rapid deployment and set-up.